1. Field of the Invention
The present invention relates generally to a gas-lift well having a controllable gas-lift valve, and in particular, to a controllable gas-lift valve which communicates with the surface and is powered using the tubing string and casing as the conductor.
2. Description of Related Art
Gas-lift wells have been in use since the 1800""s and have proven particularly useful in increasing efficient rates of oil production where the reservoir natural lift is insufficient (see Brown, Connolizo and Robertson, West Texas Oil Lifting Short Course and H. W. Winkler, Misunderstood or Overlooked Gas-lift Design and Equipment Considerations, SPE, p. 351 (1994)). Typically, in a gas-lift oil well, natural gas produced in the oil field is compressed and injected in an annular space between the casing and tubing and is directed from the casing into the tubing to provide a xe2x80x9cliftxe2x80x9d to the tubing fluid column for production of oil out of the tubing. Although the tubing can be used for the injection of the lift-gas and the annular space used to produce the oil, this is rare in practice. Initially, the gas-lift wells simply injected the gas at the bottom of the tubing, but with deep wells this requires excessively high kick off pressures. Later, methods were devised to inject the gas into the tubing at various depths in the wells to avoid some of the problems associated with high kick off pressures (see U.S. Pat. No. 5,267,469).
The most common type of gas-lift well uses mechanical, bellows-type gas-lift valves attached to the tubing to regulate the flow of gas from the annular space into the tubing string (see U.S. Pat. Nos. 5,782,261 and 5,425,425). In a typical bellows-type gas-lift valve, the bellows is preset or pre-charged to a certain pressure such that the valve permits communication of gas out of the annular space and into the tubing at the pre-charged pressure. The pressure charge of each valve is selected by a well engineer depending upon the position of the valve in the well, the pressure head, the physical conditions of the well downhole, and a variety of other factors, some of which are assumed or unknown, or will change over the production life of the well.
Referring to FIG. 1 in the drawings, a typical bellows-type gas-lift valve 310 has a pre-charge cylinder 312, a metal bellows 314, and entry ports 316 for communicating gas from the annular space outside the tubing string. Gas-lift valve 310 also includes a ball 318 that sealingly engages a valve seat 319 when valve 310 is in a closed position. When gas-lift valve 310 is in an open position, ball 318 no longer engages valve seat 319, thereby allowing gas from the annular space to pass through entry port 316, past ball 318, and through exit port 320. Several problems are common with bellows-type gas-lift valves. First, the bellows often loses its pre-charge, causing the valve to fail in the closed position or changing its setpoint to operate at other than the design goal, and exposure to overpressure causes similar problems. Another common failure is erosion around valve seat 319 and deterioration of the ball stem in the valve. This leads to partial failure of the valve or at least inefficient production. Because the gas flow through a gas-lift valve is often not continuous at a steady state, but rather exhibits a certain amount of hammer and chatter as ball 318 rapidly opens and closes, ball and valve seat degradation are common, leading to valve leakage. Failure or inefficient operation of bellows-type valves leads to corresponding inefficiencies in operation of a typical gas-lift well. In fact, it is estimated that well production is at least 5-15% less than optimum because of valve failure or operational inefficiencies. Fundamentally these difficulties are caused by the present inability to monitor, control, or prevent instabilities, since the valve characteristics are set at design time, and even without failure they cannot be easily changed after the valve is installed in the well.
Side-pocket mandrels coupled to the tubing string are known for receiving wireline insertable and retrievable gas-lift valves. Many gas-lift wells have gas-lift valves incorporated as an integral part of the tubing string, typically mounted to a pipe section. However, wireline replaceable side pocket mandrel type of gas-lift valves have many advantages and are quite commonly used (see U.S. Pat. Nos. 5,782,261 and 5,797,453). Gas-lift valves placed in a side pocket mandrel can be inserted and removed using a wireline and workover tool either in top or bottom entry. In lateral and horizontal boreholes, coiled tubing is used for insertion and removal of the gas-lift valves. It is common practice in oilfield production to shut off production of the well periodically and use a wireline to replace gas-lift valves. However, an operator often does not have a good estimate of which valves in the well have failed or degraded and need to be replaced.
It would, therefore, be a significant advantage if a system and method were devised which overcame the inefficiency of conventional bellows-type gas-lift valves. Several methods have been devised to place controllable valves downhole on the tubing string but all such known devices typically use an electrical cable or hydraulic line disposed along the tubing string to power and communicate with the gas-lift valves. It is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral with the tubing string or spaced in the annulus between the tubing string and the casing because of the number of failure mechanisms present in such a system. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhole communication system using a cable is shown in PCT/EP97/01621.
U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes a communication scheme for coupling electromagnetic energy in a transverse electric mode (TEM) using the annulus between the casing and the tubing. The system requires a toroidal antenna to launch or receive in a TEM mode, and the patent suggests an insulated wellhead. The inductive coupling of the system requires a substantially nonconductive fluid such as crude oil in the annulus between the casing and the tubing, and this oil must be of a higher density that brine so that leaked brine does not gather at the bottom of the annulus. This system does not speak to the issue of providing power to the downhole module. The invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication because it is expensive, has problems with brine leakage into the casing, and is difficult to use. Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657. Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,574,374; 5,576,703; and 5,883,516.
It would, therefore, be a significant advance in the operation of gas-lift wells if an alternative to the conventional bellows type valve were provided, in particular, if the tubing string and the casing could be used as the communication and power conductors to control and operate a controllable gas-lift valve.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes and indicative of the knowledge of one of ordinary skill in the art.
The problems outlined above are largely solved by the electrically controllable gas-lift well in accordance with the present invention. Broadly speaking, the controllable gas-lift well includes a cased wellbore having a tubing string positioned and longitudinally extending within the casing. The position of the tubing string within the casing creates an annulus between the tubing string and the casing. A controllable gas-lift valve is coupled to the tubing to control gas injection between the interior and exterior of the tubing, more specifically, between the annulus and the interior of the tubing. The controllable gas-lift valve is powered and controlled from the surface to regulate the fluid communication between the annulus and the interior of the tubing. Communication signals and power are sent from the surface using the tubing and casing as conductors. The power is preferably a low voltage AC at conventional power frequencies in the range 50 to 400 Hertz, but in certain embodiments DC power may also be used.
In more detail, a surface computer having a modem imparts a communication signal to the tubing, and the signal is received by a modem downhole connected to the controllable gas-lift valve. Similarly, the modem downhole can communicate sensor information to the surface computer. Further, power is input into the tubing string and received downhole to control the operation of the controllable gas-lift valve. Preferably, the casing is used as the ground return conductor. Alternatively, a distant ground may be used as the electrical return. In a preferred embodiment, the controllable gas-lift valve includes a motor which operates to insert and withdraw a cage trim valve from a seat, regulating the gas injection between the annulus and the interior of the tubing, or other means for controlling gas flow rate.
In enhanced forms, the controllable gas-lift well includes one or more sensors downhole which are preferably in contact with the downhole modem and communicate with the surface computer, although downhole processing may also be used to minimize required communications data rate, or even to make the downhole system autonomous. Such sensors as temperature, pressure, hydrophone, microphone, geophone, valve position, flow rates, and differential pressure gauges are advantageously used in many situations. The sensors supply measurements to the modem for transmission to the surface or directly to a programmable interface controller operating the controllable gas-lift valve for controlling the fluid flow through the gas-lift valve.
Preferably, ferromagnetic chokes are coupled to the tubing to act as a series impedance to current flow on the tubing. In a preferred form, an upper ferromagnetic choke is placed around the tubing below the tubing hanger, and the current and communication signals are imparted to the tubing below the upper ferromagnetic choke. A lower ferromagnetic choke is placed downhole around the tubing with the controllable gas-lift valve electrically coupled to the tubing above the lower ferromagnetic choke, although the controllable gas-lift valve may be mechanically coupled to the tubing below the lower ferromagnetic choke. It is desirable to mechanically place the operating controllable gas-lift valve below the lower ferromagnetic choke so that the borehole fluid level is below the choke.
Preferably, a surface controller (computer) is coupled via a surface master modem and the tubing to the downhole slave modem of the controllable gas-lift valve. The surface computer can receive measurements from a variety of sources, such as downhole and surface sensors, measurements of the oil output, and measurements of the compressed gas input to the well (flow and pressure). Using such measurements, the computer can compute an optimum position of the controllable gas-lift valve, more particularly, the optimum amount of the gas injected from the annulus inside the casing through the controllable valve into the tubing. Additional enhancements are possible, such as controlling the amount of compressed gas input into the well at the surface, controlling back pressure on the wells, controlling a porous frit or surfactant injection system to foam the oil, and receiving production and operation measurements from a variety of other wells in the same field to optimize the production of the field.
The ability to actively monitor current conditions downhole, coupled with the ability to control surface and downhole conditions, has many advantages in a gas-lift well.
Gas-lift wells have four broad regimes of fluid flow, for example bubbly, Taylor, slug and annular flow. The downhole sensors of the present invention enable the detection of flow regime. The above referenced control mechanisms-surface computer, controllable valves, gas input, surfactant injection, etc.xe2x80x94provide the ability to attain and maintain the desired flow regime. In general, well tests and diagnostics can be performed and analyzed continuously and in near real time.
In one embodiment, all of the gas-lift valves in the well are of the controllable type in accordance with the present invention. It is desirable to lift the oil column from a point in the borehole as close as possible to the production packer. That is, the lowest gas-lift valve is the primary valve in production. The upper gas-lift valves are used for annular unloading of the well during production initiation. In conventional gas-lift wells, these upper valves have bellows pre-set with a margin of error to ensure the valves close after unloading. This means operating pressures that permit closing of unloading valves as each successive valve is uncovered. These margins result in the inability to use the full available pressure to lift at maximum depth during production: lift pressure is lost downhole to accommodate the design margin offset at each valve. Further, such conventional valves often leak and fail to fully close. Use of the controllable valves of the present invention overcomes such shortcomings.
In an alternate embodiment, a number of conventional mechanical bellows-type gas-lift valves are longitudinally spaced on the tubing string in a conventional manner. The lower-most valve is preferably a bellows-type valve which aids in unloading of the well in the normal manner. The bellows-type valve""s pre-charged pressure is set normally. That is, the unloading pushes annular fluid into the tubing through successively deeper gas-lift valves until the next to the last gas-lift valve is cleared by the fluid column. Production is then maintained by gas injection through a controllable gas-lift valve located on the tubing string, which as outlined above receives power and communication signals through its connection to the tubing and a grounding centralizer. While only one controllable gas-lift valve is described, more can be used if desired, depending upon the characteristics of a particular well. If the controllable gas-lift valve fails, the production is diverted through the lowest manual valve above the controllable gas-lift valve.
Construction of such a controllable gas-lift well is designed to be as similar to conventional construction methodology as possible. That is, after casing the well, a packer is typically set above the production zone. The tubing string is then fed through the casing into communication with the production zone. As the tubing string is made up at the surface, a lower ferromagnetic choke is placed around one of the conventional tubing string sections for positioning above the bottom valve, or a pre-assembled joint prepared with the valve, electronics module, and choke may be be used. In the sections of the tubing string where it is desired, a gas-lift valve is coupled to the string. In a preferred form the downhole valve is tubing conveyed, but a side pocket mandrel for receiving a slickline insertable and retrievable gas-lift valve may also be used. With the side-pocket mandrel, either a controllable gas-lift valve in accordance with the present invention can be inserted, or a conventional bellows-type valve can be used. The tubing string is made up to the surface, where a ferromagnetic choke or other electrical isolation device such as an electrically insulating joint is again placed around the tubing string below the tubing hanger. Communication and power leads are then connected through the wellhead feed through to the tubing string below the upper ferromagnetic choke or other isolation device.
In an alternative form of the controllable gas lift well, a pod having only a sensor and communication device is inserted without the necessity of including a controllable gas-lift valve in every pod. That is, an electronics module having pressure, temperature or acoustic sensors or other sensors, a power supply, and a modem may be tubing conveyed or inserted into a side pocket mandrel for communication to the surface computer or with other downhole modules and controllable gas lift valves using the tubing and casing as conductors. Alternatively, such electronics modules may be mounted directly on the tubing (tubing conveyed) and not be configured to be wireline replaceable. If directly mounted to the tubing an electronic module or a controllable gas-lift valve may only be replaced by pulling the entire tubing string. In an alternative form, the controllable valve can have its separate control, power and wireless communication electronics mounted in the side pocket mandrel of the tubing and not in the wireline replaceable valve. In the preferred form, the electronics are integral and replaceable along with the gas-lift valve. In another form, the high permeability magnetic chokes may be replaced by electrically insulated tubing sections. Further, an insulated tubing hanger in the wellhead may replace the upper choke or such upper insulating tubing sections.